1. Field of the Invention
The present invention relates to a viscoelastic resin composition for a vibration damping material and a vibration damping material using the same. More specifically, the present invention relates to a viscoelastic resin composition capable of sufficiently absorbing a vibration energy and effectively converting the vibration energy into a heat energy, and an attachable vibration damping material which shows a vibration damping property by being attached to a vibrating portion, or a sandwich-type vibration damping material.
2. Description of the Related Art
In recent years, with the development of a transportation system and the access of a residence to factories, environmental pollution such as noises and vibration has caused social problems. Thus, there is a tendency that noises and vibration are regulated for the purpose of improving the environment of a residence near a work place. Under this circumstance, there has been a demand for providing a metallic material (i.e., a source of noises and vibration) with a vibration damping property end improving the property. Here, the vibration damping property means a function of absorbing a vibration energy of a member itself which generates noises, converting the vibration energy into a heat energy, attenuating a vibration speed or a vibration amplitude, and thereby minimizing sound reflection. Electronic parts such as those used in a computer have advanced in high performance, precision, and miniaturization. Thus, it is necessary to reduce the transmission of vibration from outside or inside the computer so as to allow these electronic parts to precisely operate.
In order to satisfy the above-mentioned demands, a vibration damping material having a composite structure in which a film of a viscoelastic resin is sandwiched between two substrates made of a steel plate, a stainless steel foil, or the like (hereinafter, referred to as a sandwich-type vibration damping material) has been proposed as material showing a vibration damping property. The sandwich-type vibration damping material has been studied and applied to oil pans of automobiles, engine covers, chutes of hoppers, stoppers of conveying apparatuses, domestic electric equipment, vibration reducing members of metal processing machines, structural members of precision machines in which the prevention of vibration is desired, and the like.
In general, the vibration damping property of the above-mentioned sandwich-type vibration damping material depends on the performance of a viscoelastic resin used in the same. This vibration damping property can be expressed in terms of a loss factor which represents a degree of conversion of an external vibration energy into a heat energy by internal friction and which corresponds to an amount showing a dynamic hysteresis loss due to vibration. When the dynamic hysteresis loss factor is measured at various temperatures and a graph showing the variation of the factor is drawn, the graph shows a peak at a certain temperature. It is known that the vibration damping material is most effectively used in the vicinity of the temperature at which the factor shows a peak.
Hitherto, the following examples of a resin layer of the sandwich-type vibration damping material have been known: a simple polyester resin (Japanese Laid-Open Patent Publication No. 50-143880); a resin composition obtained by adding a plasticizer to a polyester (Japanese Laid-Open Patent Publication No. 51-93770); a resin composition obtained by mixing an organic peroxide with a polyester (Japanese Laid-Open Patent Publication Nos. 51-41080 and 51-83640); a resin composition which is a combination of a plurality of polyesters (Japanese Laid-Open Patent Publication Nos. 62-295949 and 63-202446); a simple polyurethane foam (Japanese Laid-Open Patent Publication No. 51-91981), a simple polyamide resin (Japanese Laid-Open Patent Publication No. 56-159160); a simple ethylene-polyvinyl acetate copolymer (Japanese Laid-Open Patent Publication No. 57-34949); a resin composition obtained by adding a plasticizer and a tackifier to a polyvinyl butyral or to a combination of a polyvinyl butyral and a polyvinyl acetate (Japanese Patent Publication No. 55-27975); a copolymer of a isocyanate prepolymer and a vinyl monomer (Japanese Patent Publication No. 52-26554); copolymers disclosed in Japanese Laid-Open Patent Publication No. 60-258262, Japanese Patent Publication Nos. 39-12451 and 45-34703, and U.S. Pat. No. 4,447,493; and the like.
Another example of the vibration damping material includes an attachable vibration damping material in which a viscoelastic resin layer is formed on the surface of a base layer having high rigidity. The attachable vibration damping material exhibits a vibration damping property by being directly attached to a vibrating portion or a vibration transmitting portion. This material is used for the purpose of reducing noises end vibration of office apparatuses, domestic electric equipment, terminal apparatuses of computers, etc. This type of material is light-weight and easily used.
In general, the sandwich-type and attachable vibration damping materials are required to have a high loss factor in a wide range of temperatures. However, a conventional resin composition used for a viscoelastic resin layer does not fully satisfy this requirement. Moreover, in the sandwich-type and attachable vibration damping materials, high adhesion between the resin layer and the base layer or the vibration portion, and satisfactory durability under various circumstances are required. The conventional resin composition does not satisfy these requirements. The attachable vibration damping material is directly attached to a vibrating portion by adhering the resin layer thereof to the vibrating portion. Thus, in the case where a viscoelastic resin composition, which has been conventionally used for the attachable vibration damping material, is used for the resin layer, there are a number of problems involving heat-resistance and durability. At a high temperature, the resin composition is likely to be decomposed. As a result, serious problems are caused when the resin composition is used for an external memory device of a computer.